In recent years, many countries have pursued changes from analog to digital in the terrestrial television broadcasting. Digital broadcasts have such merits as (1) frequency resources can be effectively used, and (2) high-quality images can be provided. In digital broadcasts, it is also possible to use attachment data in the broadcast programs, where the attachment data can be reproduced in synchronization with the images or voices and is written in a language such as BML (Broadcasting Markup Language).
Now, a brief description will be provided to explain the procedure of digital-broadcasting a program composed of image data and attachment data conforming to MPEG2-TS (Moving Picture Experts Group 2-Transport Stream) defined in the ISO/IEC13818. First, an encoder encodes (compresses) image data, which is a image file in a baseband form, a DV format or the like containing images captured with a digital video recorder or the like, to an MPEG2 format defined in the ISO/IEC13818. With this encoding, the image data is packetized into a plurality of image PESs (Packetized Elementary Streams) which are each 64-byte or smaller.
A plurality of image PESs make up one access unit. A DTS (Decoding Time Stamp) is attached to each image PES to be reproduced first in each access unit. The DTS contains a value called PCR (Program Clock Reference), which is one of PCRs sequentially generated by a PCR generating apparatus of a broadcasting station and is used to manage the time when the encoded image data is decoded.
The PCRs are values “0” to “233−1” that are generated by counting based on a standard clock frequency of 90 KHz. The PCRs are used for the DTS as described above, and are used in generating the image sync information that indicates the timing with which the attachment data is to be reproduced in synchronization with the image data.
The image PESs are further packetized by the encoder into a plurality of TS packets with a fixed length of 188 Kbyte. The TS packets of the image data and the attachment data are multiplexed by a multiplexing apparatus, digitally modulated by a modulator, and are broadcast.
Meanwhile, in the terrestrial television broadcasting, a broadcast program supply system has been established. In the broadcast program supply system, a key station, or a main broadcasting station supplies local broadcasting stations with broadcast programs. The key station broadcasts a program produced by itself, and at the same time supplies the local broadcasting stations with the broadcast program. The local broadcasting stations broadcast the supplied broadcast program as has been incorporated in their broadcasting schedules.
However, as the experience in conversion from analog to digital increases, a problem in such a broadcast program supply system in digital broadcasting has been found.
That is to say, when the local broadcasting stations broadcast a broadcast program received from the key station as they are in digital broadcasting, receivers that receive and reproduce the broadcast program cannot synchronize reproduction of the attachment data with that of the image data.
The cause of the above problem is considered as follows. The digital broadcast programs transferred from the key station to the local stations are composed of image data and attachment data that should be reproduced in synchronization. The attachment data contains image sync information, which contains PCRs generated by a PCR generating apparatus of the key station. On the other hand, the transferred image data is encoded by the local stations and DTSs containing PCRs generated by the PCR generating apparatuses of the local stations are attached to the image PESs. In general, a different standard time is set by each PCR generating apparatus in each broadcasting station, where the counting of the PCRs starts based on the standard time. As a result, the key station and the local stations generate different PCRs which are also contained in DTSs and attached to image PESs. In these circumstances, the local stations fail to reproduce attachment data in synchronization with image PESs encoded by the local stations since they use the image sync information transferred from the key station as it is.